1. Field of the Invention
The present invention relates to a technology for discharging a sheet of a recording medium in an image forming apparatus.
2. Description of the Related Art
In an image forming device, a paper sheet is passed through a nip between a pressure roller and a heating roller in a fixing device to fix a toner image onto the paper. However, the paper sheet can curl when it passes through the nip. More specifically, a leading end and a trailing end of the paper sheet in its conveying direction can curl upward (see FIG. 10) due to heat applied thereto and/or ambient temperature in the fixing device. If a paper sheet with a large upward curl is discharged onto a paper discharge tray (hereinafter, “tray”) in which paper sheets are stacked, discharged paper sheets in the tray can be disrupted. More specifically, when the next paper sheet having undergone the fixing process is discharged into the tray that contains curled paper sheets by a discharge roller, the next paper sheet can push a trailing end of a stack of the paper sheets in the tray or enter through the stack to be beneath another paper sheet of the stack. As a result, some of the stacked paper sheets can be pushed out of the tray or the order of the paper sheets can be disrupted. To this end, a larger sheet discharge tray has been employed in some cases, while an additional movable sheet guide has been arranged near the discharge roller in other cases.
For example, Japanese Patent Application Laid-open No. H5-338899 discloses a technique that uses a movable sheet guide that is driven by a drive source such as a solenoid. When a leading edge of a paper sheet (hereinafter, “sheet”) is discharged onto a sheet discharge tray, the sheet is discharged with its leading end lifted upward so that the sheet will not push the leading end of a stack of sheets already in the tray. In contrast, the sheet is discharged with its trailing end pushed downward so that the trailing end of the stack is lifted up and less easily pushed by the leading edge of the sheet.
However, the approach that uses a larger sheet discharge tray is disadvantageous in requiring a larger space for the tray. The other approach as disclosed in Japanese Patent Application Laid-open No. H5-338899 is disadvantageous in that the movable sheet guide requires an additional drive source, and an additional gear or a spring, as well as additional space for these components. From another point of view, it is possible to construct a sheet discharge device compact or less expensively if a larger tray or a movable sheet guide becomes unnecessary.
Japanese Patent Application Laid-open No. 2001-270642 discloses a technique that uses sheet members that retain a sheet being discharged and are arranged on both sides of a path of the sheet. The sheet members are arranged such that downstream ends of the sheet members in a sheet conveying direction are positioned downstream relative to a nip between a pair of discharge rollers through which the sheet is to be discharged. Accordingly, because curling of the sheet at the both sides is prevented, a trailing end of a stack of discharged sheets in a sheet discharge tray is prevented from being pushed by a leading edge of the sheet in the sheet conveying direction.
However, this approach is disadvantageous in that because the sheet members are long as compared to the sheet, the sheet can be bent while the sheet is discharged.
Japanese Patent Application Laid-open No. 2000-26005 discloses a sheet finisher that resiliently deforms, even when a sheet to be discharged is thin or thick, the sheet by an appropriate amount of deformation. To attain this, the sheet finisher includes a resilient deformation roller, an arm member that causes the resilient deformation roller to be swung, and a spring that urges the arm member. However, this approach is disadvantageous in requiring additional cost and space for the resilient deformation roller and the like.
Japanese Patent Application Laid-open No. 2002-284441 discloses a sheet discharge apparatus that determines whether a sheet being discharged is curled downward. The sheet discharge apparatus includes two spatulate feelers, a downward-curl detecting sensor, and a curled-sheet-full detecting plate. The spatulate fillers are arranged on a shaft different from a shaft on which sheet discharge rollers are arranged. Whether a sheet being discharged is curled downward is determined based on an output signal of the downward-curl detecting sensor. Whether a sheet discharge tray is full with curled sheets is determined by using the curled-sheet-full detecting plate. However, this approach is also disadvantageous in requiring additional space for these components.
Japanese Patent Application Laid-open No. H11-79519 discloses a technique that uses a sheet retainer that retains a sheet being discharged. The sheet retainer is fixed at one end to a frame, which is a support of the sheet retainer, while the sheet retainer is slidably attached to the frame at the other end. The sheet retainer has, between these ends, a bent portion that protrudes toward a sheet discharge tray and that is arranged near a sheet discharge surface of the tray. However, this approach is disadvantageous in requiring additional cost and space for arranging the frame that is independent from a shaft on which a sheet discharge roller is supported.
Japanese Patent Application Laid-open No. 2002-249270 discloses a technique that uses a sheet retainer arranged at a position where the sheet retainer does not contact a leading end portion of a sheet being discharged by a discharge roller but contacts a trailing end portion of a sheet having been discharged by a sheet discharge roller onto a sheet discharge tray.
Image forming apparatuses typically include a sheet discharge device that receives a printed sheet conveyed from a fixing device and discharges the sheet by using sheet discharge rollers onto a sheet discharge tray to stack the printed sheets in the tray. In such a sheet discharge device, a sheet that exits the sheet discharge rollers is typically resiliently deformed to adjust a position where the sheet is to be placed in the tray according to the shape of the tray. By this adjustment, sheets stacked in the tray can be aligned. This resilient deformation of the sheet is typically performed by using a portion of a shaft of a drive roller or a driven roller at which the diameter of the shaft is larger than that at a nip between the sheet discharge rollers. It is also typical that a delivery member is arranged near the discharge rollers. The delivery member comes into contact with a trailing edge of a sheet being discharged and forces the sheet to exit the discharge rollers, thereby preventing a trailing edge of the sheet from remaining near the discharge rollers. Hence, the sheet can be discharged onto the tray without fail. The delivery member has typically an uneven surface and a diameter that is larger than that of the nip.
When an amount of curl of a sheet being discharged is large, the sheet can push, at its leading edge, a stack of sheets in a sheet discharge tray, causing one or more of the sheets of the stack to be pushed out of the tray or disrupting the order of the sheets in the tray. To this end, a sheet retainer that presses a portion of the stack near its trailing against a sheet discharge tray is employed in some cases so that the leading edge of the sheet being discharged will not push one or more sheets of the stack out of a tray.
However, when the sheet retainer is arranged and a trailing end portion of a sheet being discharged is deformed downward, the sheet has a wavy profile in a sheet width direction. This wavy profile prevents the sheet from coming into full contact with the large-diameter portion. In this case, it is possible that the sheet is insufficiently deformed or fails to exit the discharge rollers.
Japanese Patent No. 3323661 discloses a technique for preventing a sheet being discharged from undesirably curling or pushing an already-discharged sheet by using a protrusion. This protrusion is arranged at a substantially center on a surface of a discharge pawl in a sheet width direction. The discharge pawl comes into sliding contact with a sheet being discharged. The sheet is resiliently deformed into a wavy profile while passing through the protrusion.
Japanese Patent No. 4116973 discloses an approach for reducing an amount of curl by using a sheet retaining mechanism even when a sheet being discharged onto an internal sheet-receiving tray has a large curl due to properties of material of the sheet or its moisture absorption.